Happy Halloween with Exoskeletons

Happy Halloween! CIMI staff love to wear costumes, but not just on Halloween! We wear them every time we teach squid dissection. You can often see us dressing up like fairies, senior citizens, mad scientists, pirates, and even ghostly skeletons, which brings me to my next point. Did you know crustaceans, like crabs and lobsters, participate in Halloween every day? They wear skeletons on their outsides all the time! Unlike a costume or an internal human skeleton, called an endoskeleton, crustaceans have external skeletons, called exoskeletons. Exoskeletons are made up of chitin, a compound rich with calcium. This is similar to keratin, which comprises your nails and hair. Exoskeletons are tough, like armor, and relatively inelastic, so much like buying a new costume when you outgrow your old one, crustaceans must shed, or molt, their exoskeletons as they grow in a process scientists call ecdysis.

Ecdysis can be a pretty scary process to undergo. It is comparable to taking off your wetsuit without being able to use your hands! When an organism begins ecdysis, it enlarges the skin cells beneath its old exoskeleton and begins secreting calcium to form its new one. It then pumps in seawater to force the old exoskeleton away from its tissues, splitting it in half at the base of the carapace, or the upper body, and the tail, so it can escape. Now here comes the really scary part… In addition to shedding their outer covering, crustaceans must also shed their eye surfaces, throat and gut linings! Crustaceans have grinding teeth in their stomachs, called their gastric mill, that they use to break down their food. Since these grinding teeth are composed of chitin, the crustacean must shed them to grow a larger gastric mill for a larger stomach. It is possible during this tricky molting process to accidentally tear of an eye or a limb, or get stuck all together! How terrifying! Fortunately, crustaceans have the ability to regenerate, or regrow, their lost appendages just for this occasion. If a crustacean loses an eye, however, they must regenerate it quickly because the hormone that prevents them from molting continuously is distributed from a gland in their eye stalk. Without an eye, and without this hormone, crustaceans are unable to cease ecdysis and perform other normal body functions. Hurry, grow it back!

Escape from an old exoskeleton can take anywhere from several minutes to a half an hour, depending on the species, size, and environmental conditions. While escaping, clawed crustaceans must dehydrate and shrink their tissues enough to pull their whole claw through a hole the size of their wrist! Once all the way out, the crustacean swells with seawater, becoming up to 15% larger in size and 40-50% heavier in weight, only to release that seawater once it is done calcifying, or hardening, its new exoskeleton. This ensures that the newly molted crustacean has formed a larger exoskeleton and provided ample growing room for the future. Often, in order to speed up calcification, the crustacean eats its old exoskeleton to reabsorb some of the calcium it put into making its old armor. This may seem ghastly, but this way, the crustacean guarantees that it has enough calcium to generate its new exoskeleton.

While the crustacean hardens its armor in the next few hours after molting, it is too soft to defend itself against predators or other larger crustaceans. Newly molted lobsters are so rubbery and squishy that they are fondly referred to by fishermen as ‘jellies.’ To prevent a fight they cannot win, crustaceans tend to molt and calcify in the safety of their burrows, whether that be a hole in the sand or a deep crevice in the rocks.

With how spooky a process ecdysis can be, it is a good thing that crustaceans molt less with age as their growth slows. This is especially true for adult females, who naturally molt less because they cannot undergo ecdysis while they are laden with eggs. While an adult male lobster will molt once a year, an adult female lobster carrying eggs will molt once every two years. Regardless of sex, crustaceans grow slower, and therefore molt less, in colder waters. It is then safe to assume that crustaceans are always hoping for a colder Halloween!

Written By: Kathy Miller